12.1. Signals

12.1.1. Introduction

12.1.1.1. Finding the signal man page

Your system contains a man page listing all the available signals, but depending on your operating system, it might be opened in a different way. On most Linux systems, this will be man 7 signal. When in doubt, locate the exact man page and section using commands like

man -k signal | grep list

or

apropos signal | grep list

Signal names can be found using kill -l.

12.1.1.2. Signals to your Bash shell

In the absence of any traps, an interactive Bash shell ignores SIGTERM and SIGQUIT. SIGINT is caught and handled, and if job control is active, SIGTTIN, SIGTTOU and SIGTSTP are also ignored. Commands that are run as the result of a command substitution also ignore these signals, when keyboard generated.

SIGHUP by default exits a shell. An interactive shell will send a SIGHUP to all jobs, running or stopped; see the documentation on the disown built-in if you want to disable this default behavior for a particular process. Use the huponexit option for killing all jobs upon receiving a SIGHUP signal, using the shopt built-in.

12.1.1.3. Sending signals using the shell

The following signals can be sent using the Bash shell:

Table 12-1. Control signals in Bash

Standard key combination

Meaning

Ctrl+C

The interrupt signal, sends SIGINT to the job running in the foreground.

Ctrl+Y

The delayed suspend character. Causes a running process to be stopped when it attempts to read input from the terminal. Control is returned to the shell, the user can foreground, background or kill the process. Delayed suspend is only available on operating systems supporting this feature.

Ctrl+Z

The suspend signal, sends a SIGTSTP to a running program, thus stopping it and returning control to the shell.

Terminal settings

Check your stty settings. Suspend and resume of output is usually disabled if you are using "modern" terminal emulations. The standard xterm supports Ctrl+S and Ctrl+Q by default.

12.1.2. Usage of signals with kill

Most modern shells, Bash included, have a built-in kill function. In Bash, both signal names and numbers are accepted as options, and arguments may be job or process IDs. An exit status can be reported using the -l option: zero when at least one signal was successfully sent, non-zero if an error occurred.

Using the kill command from /usr/bin, your system might enable extra options, such as the ability to kill processes from other than your own user ID and specifying processes by name, like with pgrep and pkill.

Both kill commands send the TERM signal if none is given.

This is a list of the most common signals:

Table 12-2. Common kill signals

Signal name

Signal value

Effect

SIGHUP

1

Hangup

SIGINT

2

Interrupt from keyboard

SIGKILL

9

Kill signal

SIGTERM

15

Termination signal

SIGSTOP

17,19,23

Stop the process

SIGKILL and SIGSTOP

SIGKILL and SIGSTOP can not be caught, blocked or ignored.

When killing a process or series of processes, it is common sense to start trying with the least dangerous signal, SIGTERM. That way, programs that care about an orderly shutdown get the chance to follow the procedures that they have been designed to execute when getting the SIGTERM signal, such as cleaning up and closing open files. If you send a SIGKILL to a process, you remove any chance for the process to do a tidy cleanup and shutdown, which might have unfortunate consequences.

But if a clean termination does not work, the INT orKILL signals might be the only way. For instance, when a process does not die using Ctrl+C, it is best to use the kill -9 on that process ID:

When a process starts up several instances, killall might be easier. It takes the same option as the kill command, but applies on all instances of a given process. Test this command before using it in a production environment, since it might not work as expected on some of the commercial Unices.